It is becoming common for whole blood collected from a donor to be separated into blood component preparations, such as an erythrocyte preparation, a thrombocyte preparation, and a blood plasma preparation, stored and then provided for transfusion. Since microaggregates and leukocytes included in these blood preparations cause various side effects during blood transfusion, many methods of removing these undesirable components before blood transfusion and then performing transfusion, or performing transfusion with preparations with undesirable components having been removed after blood collection and then temporarily been stored, have been widely used.
Among methods of removing these undesirable components from blood preparations, treatment of blood preparations through a blood processing filter is most typical. As blood processing filters, filter elements with an outer edge portion that are made of nonwoven fabric, porous body or the like being internally mounted in containers by heating bonding, such as heat seal, high frequency bonding, or ultrasonic bonding have been known (e.g., Patent Literatures 1 to 5). In the heating bonded blood processing filter, the outer edge portion of the filter element is melted and integrated with the container, thereby preventing side leakage (side flow) of blood from flowing over the outer edge portion of the filter element without being filtered.
Furthermore, as another blood processing filter, what includes an inlet-side hard container element and an outlet-side hard container element which are joined with each other, and rib-shaped convexes provided for the inlet-side hard container element and the outlet-side hard container element are pressed against each other to thereby clamp the outer edge portion of the filter element has been known. In the blood processing filter using the convexes, the convexes are compressed to each other to a high density, thereby preventing side leakage (side flow) of blood. If such a type of blood processing filter is manufactured using a flexible container element, occurrence of the side flow cannot be reduced to a practical use level.